The present invention relates to method and system for controlling an engine air-fuel ratio.
An engine of an outboard motor, for example, degrades as time passes, and consequently, a state or condition of an exhaust gas from the engine gets worse and at the same time, drivability thereof, such as acceleration, also gets worse. Therefore, it has been required for the engine to perform a feedback control to monitor the state of the exhaust gas, by using an oxygen (O.sub.2) sensor or the like. The fuel injection quantity or the like is their changed according to the operating state of the engine to thereby control the air-fuel ratio, so that the exhaust gas state and the drivability of the engine may be kept in a good condition.
However, for example, in an engine used for an outboard motor, there is a possibility that the sea water may enter an exhaust gas passage to which the oxygen (O.sub.2) sensor is usually mounted. In such a situation it is difficult or impossible to use an 0.sub.2 sensor, and the feedback control may also become difficult or impossible.
Furthermore, in a case where an engine is used for an automobile, although it is possible to use an O.sub.2 sensor, the durability and reliability of the engine cannot be ensured in consideration of the degradation, variation in detecting capability, accuracy or the like of the sensor. Especially, in the high operational speed range of the engine, in order to keep the drivability thereof in a good condition, the term of an output air-fuel ratio is usually used as the air fuel ratio rather than the term of the theoretical air-fuel ratio, and therefore, it is impossible to suitably perform the feedback control mentioned above.
Furthermore, it has resulted in cost-up that an O.sub.2 sensor is provided for an engine at all times.